The news that the secret program's accidental death toll is higher raises new questions as President Obama prepares to give a major national security speech.

Members of the New York Air National guard with a drone aircraft at Hancock… (Carolyn Cole / Los Angeles…)

WASHINGTON — As President Obama prepared to deliver a major speech on national security Thursday, his administration acknowledged for the first time that it had killed four U.S. citizens — one more than previously known — in drone missile strikes in Yemen and Pakistan.

The disclosure Wednesday raised fresh questions about the secret drone campaign, a signature part of Obama's counter-terrorism effort, in which several thousand suspected terrorists, militants and others have been killed. The White House has insisted the targeting is precise and causes few accidental casualties.

In a letter to congressional leaders, Atty. Gen. Eric H. Holder Jr. said the administration had deliberately targeted Anwar Awlaki, a radical Muslim cleric who was born in New Mexico and killed in Yemen in September 2011, andhad killed three other Americans who were not targeted.

They include Samir Khan, an Al Qaeda propagandist who grew up in Queens, N.Y., and who was killed alongside Awlaki, and Awlaki's 16-year-old son, Abdulrahman, who was born in Denver and killed in Yemen two weeks after his father. All three deaths were reported by the news media, but the administration had not previously admitted its role publicly.

The fourth American was Jude Kenan Mohammad, killed in northwestern Pakistan in November 2011. His death remained so secret that he was still listed Wednesday as wanted by the FBI. Mohammad, who grew up near Raleigh, N.C., traveled to Pakistan in 2008 "to engage in violent jihad," according to a 2009 federal indictment in North Carolina.

"These individuals were not specifically targeted by the United States," Holder wrote.

Officials refused to provide details of Mohammad's death. He may have been killed in a so-called signature strike, which targets a group of suspected militants whose names are not known. The administration has not publicly acknowledged such strikes.

The development raises the stakes for a long-awaited policy speech Obama is to give Thursday at the National Defense University. He faces growing pressure to explain the rationale for secret drone strikes, especially against U.S. citizens, and to outline his strategy against a weakened Al Qaeda.

Obama, who in some cases personally approves individual drone strikes, has never revealed the legal framework his administration uses to select who is targeted and what evidence is considered. The targeted killings receive closed-door congressional oversight but no apparent judicial review.

The deaths of three Americans who weren't specifically targeted suggests the process may be less accurate than authorities have indicated. Former White House counter-terrorism chief John Brennan, who now heads the CIA, insisted in June 2011 that for almost a year, "there hasn't been a single collateral death because of the exceptional proficiency, precision of the capabilities we've been able to develop."

Obama's speech comes as some lawmakers and outside experts question whether the political backlash created by U.S. drone strikes in Pakistan, Yemen and Somalia outweighs the value of killing individual members or supporters of the Al Qaeda terrorist network, including those whom U.S. officials cannot identify.

A senior White House official said Obama "will discuss why the use of drone strikes is necessary, legal and just, while addressing the various issues raised by our use of targeted action."

The official, who asked not to be identified because the president hadn't delivered the speech, said Obama would announce that he was signing new presidential policy guidance "that lays out the standards under which we take lethal action."

Obama may also explain why the number of reported drone attacks — the official number has never been disclosed — has fallen sharply since last year. It's unclear if the decline reflects new White House sensitivity about the policy, a lack of reliable intelligence on potential adversaries, or a sign that the CIA is running out of high-value targets.

Several former senior White House aides have urged Obama to make the drone program more transparent and to shift more of the CIA's drone fleet to the military, in part to allow the CIA to return to more traditional espionage pursuits.

In a May 7 speech at Oxford University in Britain, Harold Koh, who served as the State Department's top lawyer in Obama's first term, offered a sharp critique of the drone program. The policy, he said, "has not been sufficiently transparent to the media, to the Congress and to our allies."

The result, he said, is "a growing perception that the program is not lawful and necessary, but illegal, unnecessary and out of control."